Method of fabricating rare-earth sintered magnet and method of fabricating rare-earth bonded magnet

ABSTRACT

A method of fabricating a rare-earth based sintered magnet having improved magnetic and mechanical characteristics is offered. Also, a method of fabricating a rare-earth bonded magnet having improved magnetic and mechanical characteristics is offered. The method of fabricating the rare-earth based sintered magnet is started with preparing powder of an alloy including a rare-earth element and a transition metal. The powder of the alloy is mixed with an additive. The mixture is compression molded and irradiated with microwaves to cause the powder to self-heat. As a result, the mixture is sintered.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of fabricating a rare-earth sintered magnet and to a method of fabricating a rare-earth bonded magnet.

2. Description of the Related Art

Fabrication of a magnet consisting chiefly of a rare-earth element relies on a general method of sintering. This sintering method includes various processing steps including dissolution of an alloy, thermal treatment, pulverization, press molding, nitriding (if necessary), sintering, thermal treatment, machining, and magnetization.

A bonded magnet that can be molded with great latitude at low cost is fabricated by subjecting a magnet to process steps up to thermal processing using sintering, pulverizing the thermally processed magnet, mixing the pulverized magnet with a resin (such as epoxy resin or nylon), and compression molding or injection molding the mixture. The compression molding or injection molding step yields great latitude in molding a compound at low cost.

Compounds such as SmCo₅, Sm₂Co₁₇, and Nd₂Fe₁₄B from which magnets are fabricated are sintered using a large furnace. Among various steps, a sintering step is especially important. In this sintering step, the large furnace heats the compound by means of a heater rod within the furnace in a vacuum or an inert ambient of Ar. The temperature is elevated or a high temperature is maintained by thermal conduction or radiant heat. Solid-phase and liquid-phase reactions of the molded object are made to progress. Thus, the sintered object is obtained.

However, with the large furnace, a temperature difference is produced between a location close to the heat-generating portion of the heater and a location at a large distance from the heat-generating portion. This affects the crystal grain diameters closely related to the magnetic characteristics of rare-earth magnets. Consequently, the magnetic characteristics suffer from variations.

In view of this problem, a method has been proposed which makes uniform the temperature distribution by arranging coils around a sintering furnace not using a large furnace and performing RF induction heating.

On the other hand, a compound such as rare earth-transition metal-nitride based material used as a magnetic material decomposes into nitrides of the rare-earth material and α-Fe at high temperatures exceeding 650° C. and thus sufficient magnetic characteristics are not obtained. Therefore, it has been impossible to use the compound in bulk form. Accordingly, this material for a magnet is limited to magnetic powder for a bonded magnet.

In an attempt to solve this problem, a method using plasma sintering has been proposed. In particular, the sintering time is shortened. Thermal decomposition produced during sintering is reduced to a minimum.

However, in the method of arranging the coils around the sintering furnace and performing thermal treatment by RF induction heating, large-scale equipment is required to arrange the coils around the large furnace. Furthermore, it is costly to cool the coils. Hence, this method is unsuited for mass production.

In the method using the plasma sintering, the sintering is done instantly but glow discharge is produced between magnetic particles, resulting in oversintering. This leads to increases in the diameters of the crystal grains. As a result, it has been impossible to obtain desired magnetic characteristics.

In this way, the aforementioned methods suffer from various problems. Furthermore, rare earth element-transition metal based magnets and rare earth-transition metal-nitride based magnets sintered by the above-described methods have magnetic characteristics that remain to be considerably lower than their theoretical values. Especially, the present situation is that rare earth-transition metal-nitride based sintered magnets have not been mass-produced.

SUMMARY OF THE INVENTION

The present invention has been made to solve the foregoing problems. It is an object of the invention to provide a method of fabricating a rare earth based sintered magnet having excellent magnetic characteristics (i.e., not thermally decomposed even if sintered) using a nitriding step capable of being performed in a reduced time. It is another object of the invention to provide a method of fabricating a rare earth based bonded magnet having excellent magnetic characteristics (i.e., not thermally decomposed even if sintered) using a nitriding step capable of being performed in a reduced time.

One embodiment of the present invention provides a method of fabricating a rare-earth sintered magnet, comprising the steps of: preparing powder of an alloy containing rare-earth element and transition metal; molding the powder of the alloy into a molded article having a desired form; irradiating the molded article with microwave in a vacuum or in an inert gas and sintering the molded article; and heating the molded article in an inert gas.

In one feature of the invention, in the irradiating step, the molded article is irradiated with microwave in nitrogen gas for nitriding and sintering.

In another feature of the invention, the microwave has a frequency higher than 1 GHz and lower than 30 GHz.

In a further feature of the invention, the powder of the alloy is made of particles having an average particle diameter of 2 to 90 μm.

In a yet other feature of the invention, in the irradiating step, a pressure of the nitrogen gas is from 0.1 to 5 MPa.

An other embodiment of the present invention provides a method of fabricating a rare-earth bonded magnet, comprising the steps of: pulverizing a rare-earth sintered magnet fabricated by the method of the present invention into powder of a rare-earth sintered magnet having an average particle diameter of 5 to 90 μm; and mixing the powder of the rare-earth sintered magnet with a resinous binder or a metal binder and compression molding or injection molding the mixture.

According to the present invention, the powder of the rare-earth magnet is allowed to self-heat quickly and selectively. The whole sample can be elevated in temperature uniformly. Therefore, the powder of the rare-earth magnet can be sintered instantly. The heating time can be shortened. This in turn can prevent evaporation of the rare-earth element. Hence, a magnet having a desired composition can be obtained.

Furthermore, only the sample is heated. Surroundings of the sample are not elevated in temperature. Consequently, after the sintering, the rate at which the sample is cooled can be increased. This in turn achieves a sintered rare-earth based magnet having excellent magnetic characteristics having no depositions. In addition, the step of homogenizing the quality of the sintered object produces a uniform phase that is important for the coercive force of the sintered rare-earth based magnet.

According to the present invention, nitriding and sintering can be carried out at the same time by irradiating the molded object within an ambient of nitrogen with microwaves. In consequence, the time taken to perform the nitriding step can be shortened. Moreover, the molded object can be nitrided uniformly up to its inner depths, thus producing a rare-earth magnet having high magnetic characteristics. Nitrogen atoms can be moved into stable sites between crystal lattice points by performing the homogenizing step. Consequently, a stable rare-earth sintered magnet having high magnetic characteristics can be obtained.

According to the present invention, microwaves applied in the sintering or nitriding step have a frequency higher than 1 GHz and lower than 30 GHz. In consequence, the molded object can be sintered uniformly instantly without producing glow discharge. In addition, the phenomenon that solid-phase diffusion progresses preferentially while leaving behind non-nitrided local areas is suppressed. Additionally, the molded object can be nitrided uniformly up to its inner depths.

According to the invention, grains of the powder of the alloy have an average grain diameter of 2 to 90 μm. Consequently, the powder of the rare-earth magnet is suppressed from becoming smaller particles each having a single magnetic domain. In addition, oxidation and excessive nitriding of the powder can be suppressed. The powder of the alloy can be uniformly nitrided.

According to the invention, the pressure of the ambient used in the nitriding step is set to 0.1 to 5 MPa. Therefore, the powder of the alloy can be nitrided uniformly. If excessive pressure were used, the alloy would be overnitrided, resulting in amorphous state.

According to the invention, a rare-earth bonded magnet is fabricated, using powder of a rare-earth magnet and a resinous binder or a metal binder. As a consequence, a bonded magnet having excellent magnetic characteristics can be obtained.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic perspective view of equipment for implementing a method of fabricating a magnet in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic perspective view of equipment for implementing a method of fabricating a magnet in accordance with one embodiment of the present invention. The equipment includes a microwave-generating tube 1 producing microwaves. The tube 1 is connected with an applicator 3 via a waveguide 2. The microwaves generated by the microwave-generating tube 1 are transmitted to the applicator 3 through the waveguide 2. An isolator 4 is mounted in the waveguide 2. The isolator 4 transmits the microwaves in the waveguide 2 only toward the applicator 3, and absorbs microwaves transmitted in the reverse direction.

A sample 5 is placed within the applicator 3 and irradiated with the microwaves. The applicator 3 is a closed metal container and designed to prevent leakage of the microwaves to the outside. A gas supply source 6 for introducing an inert gas such as nitrogen is connected with the applicator 3. Furthermore, a pump 7 for venting the internal ambient is connected with the applicator 3.

The sample 5 is in contact with a thermocouple 8. When the sample 5 is irradiated with the microwaves, the temperature of the sample 5 varies. This variation in temperature can be measured. A pressure gauge 9 is mounted to the applicator 3 and can measure the internal pressure. The microwave-generating tube 1, gas supply source 6, pump 7, thermocouple 8, and pressure gauge 9 are connected with a controller 10 and under control of this controller. The controller 10 can control the ambient inside the applicator, the pressure, and temperature rise of the sample.

A magnetron, gyrotron, krystron, or the like can be used as the microwave-generating tube 1.

A method of fabricating a rare-earth based sintered magnet made essentially of a rare earth element and a transition metal (herein after may be abbreviated R-TM based magnet) according to the invention is described below step by step. R (rare earth) is at least one rare-earth element. TM is at least one transition metal element.

(1) Powder of Rare Earth Based Magnet

The rare-earth element constituting the R-TM based alloy according to the present invention is preferably selected from Y (yttrium) and lanthanoids (La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). Especially, where Pr, Nd, or Sm is used, the magnetic characteristics can be enhanced greatly. Furthermore, where two or more rare-earth elements are combined, the residual magnetic flux density and coercive force can be improved.

In particular, powder of Sm—Co based magnets such as of SmCo₅ and Sm₂Co₁₇ can be used. Powder of Nd—Fe based magnets such as of Nd₂Fe₁₄B can also be used. Furthermore, powder of a magnet of Sm—Fe—N or Nd—Fe—N based materials can be used. More specifically, the first fundamental component is a rare earth based material consisting mainly of Sm or Nd. The second fundamental component is a transition metal based material consisting mainly of Fe. The third fundamental component is an interstitial element based material consisting mainly of N. Two or more kinds of the above-described powders of the rare-earth magnets can be mixed.

In the case of a general dissolution casting, with respect to the aforementioned powder of a rare-earth magnet of R-TM or R-TM-N based material, the rare-earth metal and transition metal are mixed at a given ratio and RF dissolved in an ambient of an inert gas. The obtained alloy ingot is thermally treated and pulverized into grains of a given grain size using a pulverizer such as a jaw crusher, jet mill, or attritor. Thus, the powder is prepared.

During this dissolution, if C and B are contained as unavoidable impurities, no problems take place.

Preferably, the grains of the powder of magnet preferably have an average grain diameter of 2 to 150 μm. Where the average gain diameter is less than 2 μm, the powder is easily oxidized. Furthermore, where the powder is molded, the grains coagulate during orientation. This makes it impossible to improve the magnetic characteristics. The magnetic characteristics remain poor. Where the average gain diameter is in excess of 150 μm, when the powder is molded while aligning the directions of magnetizations by application of a magnetic field, the particles are not aligned in the desired direction of magnetization. This will deteriorate the magnetic characteristics.

(2) Additive

No restrictions are placed on the used additive. Surface active agent, coupling agent, lubricant, mold release agent, fire retardant additive, stabilizing agent, inorganic filler, and pigment can be used as the additive. The additive should show fluidity to permit filling into the mold. The additive should show lubrication to permit the directions of magnetizations to be aligned by applying a magnetic field. The additive should permit easy release of the molded object from the mold. The additive should show improved intensity to improve handling of the molded object. The additive should permit adjustment of the density for adjustment of the contraction coefficient of the sintered object. Plural kinds of additives may be combined in use.

(3) Mixing

Preferably, the powder of a molded magnet is prepared by mixing and dispersing the powder of the above-described rare-earth based magnet and additive using an attritor, Henschel mixer, or V-blender. A sintered rare-earth magnet having good magnetic characteristics, especially a well-oriented, sintered rare-earth magnet, can be obtained.

(4) Compression Molding (Molding Step)

A press machine having a mold equipped with an electromagnet for applying a magnetic field is prepared. Using the press machine, powder of a molded magnet is filled into the mold. The powder is compression molded within a magnetic field having a strength of more than 10 kOe (kilo-oersteds) or in the absence of a magnetic field at a pressure of more than 10 tons.

If the strength of the magnetic field is less than 10 kOe, the powder of the rare-earth magnet is not aligned in the direction of magnetization. Therefore, the strength of the magnetic field needs to be greater than 10 kOe.

(5) Sintering

When the compression molding is completed, the powder of the rare-earth based magnet is sintered by irradiating the molded object with microwaves in the present embodiment. The powder of the rare-earth magnet is allowed to self-heat selectively and quickly in this way. The temperature is elevated to 750 to 1200° C. in several minutes. Heat produced by the self-heating of the powder of the rare-earth magnet sinters the powder of the magnet instantly. At this time, the whole sample is elevated in temperature uniformly. Consequently, a high-strength, sintered rare-earth based magnet having no unsintered portions can be obtained. Because the temperature can be elevated to the desired temperature in several minutes by microwave irradiation, the processing time can be shortened. Furthermore, oxidization and decomposition of the powder of the alloy are prevented. Deposition of impurities can be suppressed.

Preferably, the microwaves applied to the molded object has a frequency higher than 1 GHz and lower than 30 GHz. Where the frequency is lower than 1 GHz, arc discharge tends to be produced. Where the frequency is higher than 30 GHz, the temperature is elevated above the desired temperature. Preferably, the ambient is a vacuum or an ambient of an inert gas to suppress oxidation of the magnet.

Where the sintering and nitriding of the powder of the rare-earth magnet are carried out simultaneously, the pressure of nitrogen is preferably 0.1 to 5 MPa. Where the pressure is less than 0.1 MPa, nitrogen atoms do not enter into the grains but stay on the surfaces. Where the pressure is in excess of 5 PMa, excessive nitriding occurs on the surface of the grains.

Preferably, the nitrided powder of the rare-earth magnet consists chiefly of an R-TM based material. Furthermore, powders of rare-earth magnets made of Sm—Fe based material or Nd—Fe based material can be used.

Although the powder of the magnet can be used as powder for a bonded magnet by terminating the process at the nitriding step, if the average grain diameter is in excess of 100 μm, nitriding occurs only at the surface. The desired magnetic characteristics cannot be obtained. Therefore, in an ambient of nitrogen under pressure, nitrogen atoms are shifted into stable locations between crystal lattice points by performing sintering and thermal treatment for homogenization of the quality. An Sm—Fe—N based sintered magnet having better magnetic characteristics can be obtained.

Obviously, powder of a bonded magnet having excellent magnetic characteristics can be fabricated by chemically or mechanically pulverizing the Sm—Fe—N based sintered magnet into grains having an average grain diameter of 5 to 90 μm, mixing the grains with a resin or low-melting-point metal, and molding the mixture. The present invention can be similarly applied to powder of a rare-earth magnet made of other material.

(6) Cooling

After the microwave irradiation, the molded object obtained by sintering the powder of the rare-earth based magnet is cooled. That is, when microwave irradiation is terminated, the powder of the rare-earth magnet itself cools quickly but is inevitably oxidized somewhat. We have attempted to use a method consisting of cooling the powder while reducing the power of the microwave irradiation. However, where the power is lower than a certain level, an oxidation process occurs preferentially. The magnetic characteristics are slightly deteriorated. Therefore, vacuum pumping is required. Alternatively, it is required that the powder be cooled down to room temperature within an inert gas such as nitrogen and argon gas. In some cases, it is desired to use external cooling in combination.

The above-described embodiment yields the following advantages.

(1) In the above embodiment, the mixture of powder of a rare-earth magnet and an additive is compression molded, and the molded object is irradiated with microwaves. Consequently, the powder of the rare-earth magnet can be selectively allowed to self-heat. The powder of the magnet can be sintered. Consequently, the whole sample can be elevated in temperature uniformly, thus eliminating unsintered portions. Hence, the mechanical strength can be improved.

Because the mechanical strength can be improved by eliminating unsintered portions, the magnetic characteristics can be improved by increasing the amount of the rare-earth magnet material in the powder of the sintered magnet. Additionally, heating of the powder of the rare-earth magnet can sinter the powder of the rare-earth magnet instantly and so the processing time can be shortened. Furthermore, oxidation and decomposition that would normally be caused by prolonged heating can be prevented. Deposition of impurities can be suppressed. In this way, deterioration of the magnetic characteristics can be prevented.

(2) In the above embodiment, the molded object obtained by sintering the powder of the rare-earth magnet is cooled in a vacuum or within an inert gas. Therefore, oxidation of the powder of the rare-earth magnet is suppressed. Good magnetic characteristics can be maintained.

(3) In the above embodiment, the frequency of the microwaves applied to the molded object is set to a range of from 1 GHz to 30 GHz. Therefore, arc discharge that tends to be produced at lower frequencies can be suppressed. If the frequency is too high, the molded object will be heated to above the desired temperature. The molded object can be heated within the desired temperature range.

(4) In the above embodiment, the powder of the rare-earth magnet can be nitrided and, at the same time, sintered by irradiating the molded object with microwaves within an ambient of nitrogen under a pressure of 0.1 to 5 MPa. Therefore, the processing time can be shortened compared with the case where nitriding and sintering are carried out separately.

(5) In the above embodiment, the average grain diameter of the powder of the rare-earth magnet is set to 2 to 150 μm. For this reason, oxidation of the magnet due to increased surface area of the magnet is suppressed. When the powder is molded while aligning the directions of magnetizations, the grains can be aligned in the desired direction of magnetization.

The rare earth based sintered magnet fabricated by the method of the above-described embodiment is chemically or mechanically pulverized into powder of a rare earth based magnet consisting of grains having an average grain diameter, for example, of 5 to 90 μm. The powder of the rare earth based magnet is mixed with a resinous binder or metal binder together with an additive and compression molded or injection molded. The resinous or metal binder in the molded object may be cured or sintered to fabricate a rare-earth bonded magnet. It is possible to fabricate a rare-earth bonded magnet having excellent magnetic characteristics that is not thermally decomposed can be fabricated. 

1. A method of fabricating a rare-earth sintered magnet, comprising the steps of: preparing powder of an alloy containing rare-earth element and transition metal; molding the powder of the alloy into a molded article having a desired form; irradiating the molded article with microwave in a vacuum or in an inert gas and sintering the molded article; and heating the molded article in an inert gas.
 2. A method of fabricating a rare-earth sintered magnet as set forth in claim 1, wherein, in the irradiating step, the molded article is irradiated with microwave in nitrogen gas for nitriding and sintering.
 3. A method of fabricating a rare-earth sintered magnet as set forth in claim 1, wherein the microwave has a frequency higher than 1 GHz and lower than 30 GHz.
 4. A method of fabricating a rare-earth sintered magnet as set forth in claim 2, wherein the microwave has a frequency higher than 1 GHz and lower than 30 GHz.
 5. A method of fabricating a rare-earth sintered magnet as set forth in claim 3, wherein the powder of the alloy is made of particles having an average particle diameter of 2 to 90 μm.
 6. A method of fabricating a rare-earth sintered magnet as set forth in claim 4, wherein the powder of the alloy is made of particles having an average particle diameter of 2 to 90 μm.
 7. A method of fabricating a rare-earth sintered magnet as set forth in claim 2, wherein a pressure of the nitrogen gas is from 0.1 to 5 MPa.
 8. A method of fabricating a rare-earth bonded magnet, comprising the steps of: pulverizing a rare-earth sintered magnet fabricated by the method set forth in claim 1 into powder of a rare-earth sintered magnet having an average particle diameter of 5 to 90 μm; and mixing the powder of the rare-earth sintered magnet with a resinous binder or a metal binder and compression molding or injection molding the mixture.
 9. A method of fabricating a rare-earth bonded magnet, comprising the steps of: pulverizing a rare-earth sintered magnet fabricated by the method set forth in claim 2 into powder of a rare-earth sintered magnet having an average particle diameter of 5 to 90 μm; and mixing the powder of the rare-earth sintered magnet with a resinous binder or a metal binder and compression molding or injection molding the mixture.
 10. A method of fabricating a rare-earth bonded magnet, comprising the steps of: pulverizing a rare-earth sintered magnet fabricated by the method set forth in claim 7 into powder of a rare-earth sintered magnet having an average particle diameter of 5 to 90 μm; and mixing the powder of the rare-earth sintered magnet with a resinous binder or a metal binder and compression molding or injection molding the mixture. 